The present invention relates to a device for capacitive measurement between an object and a array of electrodes. It finds its application in particular in the general field of 2D capacitive touch surfaces and 3D capacitive detection used for human-machine interface commands.
Increasingly, devices used for communication and for work utilize a touch command interface such as a pad or a screen. There can be mentioned for example mobile phones, smartphones, electronic notebooks, PC, mice, touch screens, widescreens, etc.
A large number of these interfaces utilize capacitive technologies. The touch surface is equipped with conductive electrodes linked to electronic means making it possible to measure the variation of the capacitances created between electrodes and the object to be detected in order to perform a command.
The current capacitive techniques most frequently utilize two layers of conductive electrodes in the form of rows and columns. The electronics measure the coupling capacitances existing between these rows and columns. When a finger is very close to the active surface, the coupling capacitances close to the finger are altered and the electronics can thus locate the 2D position (XY) in the plane of the active surface.
This technology makes it possible to detect the presence and the position of the finger through a dielectric. This technique has the advantage of obtaining a very high resolution on the location in the plane XY of the sensitive surface of one or more fingers. These techniques nevertheless have the drawback of only detecting a contact with the object or detection in very close proximity but not exceeding a few mm. It is difficult to perform touch commands with thick gloves (ski gloves, motorcycle gloves, etc.), with long fingernails or with a stylus. The low sensitivity of the capacitive electrodes does not allow a command to be initiated through a thick dielectric.
It is also impossible to detect the position and the number of fingers holding the portable device in order to deduce the type of hand (left or right) and the possible screening of the screen.
There are also more recent techniques allowing measurement of the absolute capacitance created between the electrodes and the object to be detected. This technique is similar to the techniques known as self-capacitance. There can be mentioned for example patent FR2756048: Floating capacitive measuring bridge, patent FR2893711: Device and Method for Capacitive Measurement by a Floating Bridge, or patent FR2844349: Proximity Detector Comprising Capacitive Sensor. These techniques make it possible to obtain a measurement of the inter electrode-object capacitance with a very high resolution and to detect for example a finger at several cm or even at 10 cm distance. The spatial detection takes place in 3 dimensions XYZ but also by touch within the plane XY. This time it is possible to initiate a command with a glove or through any type of thick dielectric.
These techniques are possible by utilizing therefore an absolute-capacitance measurement electronics in order to be able to detect from the greatest possible distance the position of the object(s) within the space close to the active surface (above and at the periphery of the touch screen). The ideal being to cover the entire surface of the touch screen with capacitive electrodes. These electrodes are linked to an electronic unit in order to convert the capacitance created between each electrode and the object or objects to be detected.
In order to carry out an absolute measurement of these capacitances, it is necessary to eliminate all the parasitic capacitances likely to appear outside the detection zone, i.e. between the electrodes and the electronic circuit such as for example that created by the linking track of each electrode, the underneath of the electrodes, the ribbon cables between the touch screen and the electronics, the input of the electronic circuit, etc.
A large portion of these parasitic capacitances can be suppressed by the use of a guard, the potential of which has substantially the same value as that of the electrodes as described in the patent by Rozière FR2756048.
However, the detection in a volume at a long distance has the drawback of the detection of any object close to the panel but outside its surface. This can limit the possibilities of command or reduce the visible surface of the panel or initiate commands unintentionally.
To this end, it can be provided that the entire surface of the touch screen is only equipped with electrodes without apparent electrical link in order to avoid the surrounding object or objects such as for example the ends of the fingers of the hand holding the portable device being detected as wanted objects.
A solution consists of utilizing a multi-layer capacitive touch screen such as for example a Printed Circuit Board (PCB). The capacitive electrodes are deposited on the outer surface on the side of the object to be detected and all the linking tracks are situated below the electrodes at the level of a lower layer. These tracks are linked to the electrodes using metallized holes via the electrodes layer. All the tracks are connected to the electronics but are guarded until reaching the connection (a guard layer is situated below the linking tracks). Thus, the electrodes guard the tracks by utilizing an electronic unit for example a floating-bridge unit as described in patent FR2756048.
A difficulty however arises for incorporating this function into a telephone, smartphone, GPS, touch screen or any device equipped with this type of touch surface and a screen.
These surfaces of capacitive electrodes must be equipped with transparent electrodes in order to be capable of allowing the light emitted by the display located below the touch screen to pass through. In general, the electrically conductive electrodes are made from Indium Tin Oxide (ITO). This material has good optical and electrical properties. For technical reasons and those involving the manufacturing process and optical quality, it is not possible to use metallized holes and all the capacitive electrodes must be linked to the outer circuit at the sensitive surface using a transparent track only situated on the same layer as these electrodes.